1. Field of the Invention
The present invention relates to broadband communication systems and particularly to systems for transmission and reception of pseudorandom noise sequences and spread spectrum signals, and to code division multiple access systems with application to satellite and terrestrial communications.
2. Description of the Related Art
Spread spectrum systems have been used for many years in digital communications. A spread spectrum signal consists of a baseband message signal modulated onto a carrier and thereafter spread in frequency by a pseudorandom noise sequence ("PN sequence" or "PNS"), independent of the message signal itself. The receiver then recovers the message signal by using a replica of the PN sequence. The main advantages of spread spectrum systems are good interference and noise rejection, low power density, ability to access multiple channels (such as in code division multiple access (CDMA) systems), high resolution ranging, and message protection. The ratio of the bandwidth of the PNS to that of the message signal, called the processing gain, determines the merit of the system.
Typical block diagrams of a spread spectrum transmitter and receiver are found in J. K. Holmes, Coherent Spread Spectrum Systems (Wiley 1982), and reproduced as FIGS. 1a and 1b. In the transmitter in FIG. 1a, a digital message signal transmitted at bit rate B is provided to coder 104. This coder encodes the data bits into codewords for transmission and can be a block coder or a convolutional coder as described in G. C. Clark and J. B. Cain, Error-Correction Coding for Digital Communications (Plenum Press 1981). PNS modulator 106 modulates (or spreads) the coded signal with a PN sequence from PNS generator 110. Carrier frequency generator 112 generates a carrier signal that is modulated by the spread coded signal in carrier modulator 108. The PN sequence is a digital signal made up of "chips" and whose chip interval or chip period is much smaller than the data bit period (thus the bandwidth of the PN sequence is much greater than that of the data signal). The resulting signal is amplified by amplifier 120 and transmitted by antenna 130.
As depicted in FIG. 1b, the transmitted signal is received by antenna 150 and amplified by amplifier 160. The received signal is provided to carrier restoration and phasing module 166 which provides a local oscillator signal which is phase synchronized to the carrier signal. The local oscillator signal is used to demodulate the carrier from the received signal in coherent detector 162. The resulting demodulated signal is provided to tracking and acquisition (T&A) synchronism device 168 to establish synchronization between the demodulated signal and a local PNS generator in T&A synchronism device 168 which generates a replica of the PNS that was used in the transmitter. In the acquisition stage, a coarse alignment between the replicated PNS and the demodulated signal is performed using serial and/or sequential search, sequential estimation, universal timing, or matched filter algorithms. These techniques are described in various references, one of which is R. C. Dixon, Spread Spectrum Systems with Commercial Applications (Wiley 1994). The acquisition stage brings the replicated PNS and the demodulated signal within half a chip interval of each other. Once the demodulated signal is acquired, the two signals are tracked, generally using a delay-lock loop. See, e.g., J. J. Spilker, Digital Communications by Satellite (Prentice Hall 1977). Once synchronized, T&A synchronism device 168 outputs the synchronized replicated PNS to PNS demodulator 164 which further demodulates (despreads) the modulated signal to produce a baseband coded data signal which is provided to decoder 170. Decoder 170 contains circuitry to extract the clock signal, to determine whether code bits are high or low, to synchronize the code frames, and to convert coded bits into message data bits.
A two-channel version of this spread spectrum system is described in U.S. Pat. No. 5,414,728 to Zehavi, and depicted in FIGS. 2a-2c. In the transmitter of FIG. 2a, two message signals, User A and User B, are transmitted at bit rate B in I (in-phase) and Q (quadrature phase) channels, respectively, or a single user at bit rate 2B is demultiplexed in demultiplexer 202 and transmitted in the I and Q channels. The two signals are then respectively supplied to I and Q channel coders 204a and 204b. Each channel is spread by a separate PN sequence, the I-channel by PNS.sub.I generated by PNS.sub.I generator 210a, and the Q-channel by PNS.sub.Q generated by PNS.sub.Q generator 210b. In PNS modulators 206a and 206b, the coded signals modulate the respective PNS, the result being provided to I-channel carrier modulator 208a and Q-channel carrier modulator 208b, respectively. An in-phase carrier signal produced by carrier frequency generator 212 is modulated (usually by binary phase shift keying or BPSK) in I-channel carrier modulator 208a by the spread coded signal to produce an I-channel signal that is provided to summing amplifier 220. The in-phase carrier signal is phase shifted by 90.degree. in phase shifter 214 and then BPSK-modulated in Q-channel carrier modulator 208b by the spread coded signal to produce a Q-channel signal which is added to the I-channel signal in summing amplifier 220 and provided to antenna 230 for transmission.
The receiver of FIG. 2b processes the two channels, receiving the transmitted signal in antenna 250 and amplifying it in amplifier 260. As before with the single channel system depicted in FIG. 1b, the carrier signal is restored in carrier frequency restoration and phasing module 266. Carrier restoration module 266 provides in-phase and quadrature-phase carrier signals which are used to demodulate the received signal in coherent detector 262 into received I and Q component signals. Each of these is used in T&A synchronism device 268 to acquire and track the received signals and to produce a timing signal which is provided to PNS.sub.I generator 265a, and PNS.sub.Q generator 265b. PNS.sub.I generator 265a replicates PNS.sub.I to despread the received I-channel signal, and PNS.sub.Q generator 265b replicates PNS.sub.Q to despread the received Q-channel signal. In PNS demodulator 264, shown in more detail in FIG. 2c, each channel is split into subchannels. One subchannel from each channel is then multiplied with PNS.sub.I in multipliers 284a and 284b, and the other subchannel is multiplied with PNS.sub.Q in multipliers 282a and 282b. The four products are accumulated in accumulators 286a,286b,288a, and 288b for one PNS period (illustrated by switches 290a,290b,292a, and 292b and delay elements 294a,294b,296a, and 296b). The accumulated results from the subchannels in the I and Q channels that were multiplied by PNS.sub.I are provided to phase rotator 298a, and the integrated results of the subchannels in the I and Q channels that were multiplied by PNS.sub.Q are provided to phase rotator 298b. These phase rotators estimate the data sequences that were transmitted in each channel.
This prior art system is limited in its spectral efficiency, thus limiting the number of users or the data rate of the system or both. Spectral efficiency is defined by ##EQU1## where B.sub.m is the data rate of the information signal that modulates the PN sequences, .DELTA.F is the occupying frequency range, and N is the number of subscriber stations (information streams) working within the same frequency range at the same time. In orthogonal CDMA (OCDMA), it is necessary to allocate for each subscriber station two PN sequences, each sequence being of signal base (length) D. Thus, ##EQU2## In this prior art system, B.sub.m =B, the data rate of the information signal provided to PNS modulators 206a and 206b in FIG. 2a. Substituting into the spectral efficiency equation, ##EQU3## and .gamma.=1 bit/sec/Hz. It is an object of the present invention to double the spectral efficiency of a spread spectrum system by decreasing the bit rate of the information signal provided to the PNS modulator and by using two PN sequences on both the I and Q channels.